Wireless transmission systems are well known in the art. As wireless networks have become more pervasive and product manufacturing costs have made the demand for wireless phones and two-way radios steadily increase.
When transmitting a signal, these devices take an input signal, modulate it using one or more known modulation schemes, and amplify the signal for transmission. The power amplifier takes the modulated signal as its input and amplifies it using a constant or variable amplification technique. Ideally, this process yields an amplified copy of the amplifier's input signal across the signal's spectrum. In practice this type of linear amplification is difficult to obtain.
The output power of a power amplifier is generally a direct function of its supply power, that is, a higher supply power will yield greater amplification. However, the level of the input signal, the signal to be amplified will also affect the output power level of the amplified signal independent of supply power. Thus, in systems in which it is desirable to maintain a power amplifier at or near its saturation level, it becomes necessary to modify or modulate the power level of the amplifier's supply voltage.
One example of an apparatus that utilizes variable supply voltage in an RF transmitter is an envelope elimination and restoration (EER) amplifier. An EER amplifier generates a signal component representative of an input signal's envelope and couples the signal to the supply input of a power amplifier. The EER amplifier will also generate a signal component representative of the input signal's phase and couples that signal to the input of the power amplifier. As a result, the component of the input signal representative of the signal's phase will have its amplitude adjusted corresponding to the value of the component of the input signal representative of the signal's envelope, which is coupled to the power supply input of the power amplifier.
Transmitters that use EER amplifiers are known to exhibit significant intermodulation distortion levels due to the finite bandwidth in the envelope path. This distortion reduces the overall capability of the EER technique and limits its usefulness to narrowband signals. As a result, EER-based transmitters have seen only limited use.